The present disclosure relates to characterizing ion-exchanged glasses, and in particular relates to apparatus and methods for measuring mode spectra for ion-exchanged glasses having a steep and shallow near-surface refractive index region.
Certain types of glasses and glass-ceramics can be chemically strengthened by an ion exchange process. The strengthening is due to the formation of a near-surface compression layer that usually creates stress-induced birefringence. The birefringence in turn corresponds to a polarization-dependent change in refractive index profile in the glass. The ion exchange process typically alters the refractive index profile with a shape that corresponds to the concentration of the ions diffused into the glass. The stress and the birefringence are linearly related through the stress optic coefficient.
There is increasing commercial interest in chemically strengthened glasses with anti-microbial (AM) surface properties. Such glasses can be fabricated using a double ion-exchange (DIOX) process wherein a first ion exchange (IOX) process is performed for strengthening. The IOX process involves a larger alkali ion such as K+ that exchange for a smaller alkali ion, such as Na+ or Li+ present in the original base (bulk) glass. The first IOX process is followed by a second IOX process that includes an anti-microbial ion, such as Ag+. In some cases, both exchanging ions (i.e., larger alkali ions and anti-microbial ions) can be supplied simultaneously. The resulting refractive index profile includes a steep and shallow near-surface region followed by a less-steep and deeper region.
Optical methods of characterizing ion-diffused glasses using prism coupling and the analysis of the resulting mode spectra are known in the art. However, prior-art prism-coupling systems and methods cannot adequately measure the mode spectra of steep and shallow near-surface regions of a refractive-index profile.
In particular, in the prior art, the range of refractive index obtained through chemically strengthening ion exchange and measured by high-resolution mode-spectra measurement systems is usually less than about 0.02 RIU (refractive-index units). This limited range allows prior art systems and methods to take advantage of a linear approximation between the measured mode spectra positions at the photodetector and the mode effective index. However, as the index profiles attributable to anti-microbial glass often have a range of refractive index exceeding or substantially exceeding 0.03 RIU, the linear approximation becomes less accurate and leads to systematic errors in the calculated index and stress profiles, especially at and near the substrate surface.
Even more importantly, prior-art systems for measuring stress cannot detect the optical modes of propagation in the steep near-surface region of the index profile, or in some cases cannot detect them with adequate precision for practically useful stress measurements. A different-type prism-coupling system based on a laser source and using a rotating prism and piston applied to the substrate can be used to detect the modes of the shallow and steep index of the index profile, but they do not have the precision needed for stress measurement, nor the resolution to properly resolve the some of the modes that propagate primarily in the less-steep and deeper region of the refractive index profile.